Generally, an underwater vehicle (for example, an Underwater Autonomous Vehicle (UAV), a torpedo, or a generic propulsion unit submerged in the sea) comprises a shaped casing elongated along an axis and designed to move in an underwater environment under the thrust of a propulsion system housed inside the casing and powered by an energy source that is also housed inside the casing.
Such underwater vehicles are provided with hydraulic circuits in which fluids flow that must be cooled during operation of the underwater vehicle.
For example, it the energy source were provided by an electrochemical battery, the fluid to be cooled would be the electrolyte used in the electrochemical process for the production of electrical energy.
In particular, electrochemical batteries (see patent application PCT WO2005/053068 for example) comprise:                a main electric battery of the electrochemical type in which a casing defines a main chamber housing an anhydrous electrolyte (for example, sodium hydroxide) and is provided with members for the control led intake of a flow of water from the marine environment to form a liquid electrolyte, following execution of a water-intake command, and members to discharge emissions, typically gases, produced by the chemical reaction of the main battery, into the marine environment; the main chamber houses a system for recirculating the fluid electrolyte (using an electric pump) to a plurality of electrochemical cells housed in the main chamber;        an auxiliary electric battery that can be activated by remote control to open the controlled intake members and power the electric pump so as effect the formation and recirculation of the electrolyte.        
The recirculation system generally comprises a thermostatic mixing valve that allows adjusting the flow of the fluid electrolyte fed to the electrochemical cells in order to keep the temperature of the electrolyte equal to a target value. This thermostatic valve, generally three-way, has a first, inlet that receives electrolyte directly from the main chamber, a second inlet that communicates with the outlet of a heat exchanger to which electrolyte in the main chamber is fed, and an outlet communicating with an inlet of the electrochemical cells.
According to patent application PCT WO2005/053068, the heat exchanger is made from a short pipe arranged in contact with an internal face of the tubular wall of the casing formed by the body of the torpedo in such a way that part of the heat of the electrolyte can be exchanged with the seawater that surrounds the torpedo.
However, the heat exchange achieved by known devices is quite limited; for this reason, the temperature difference between the fluids fed to the first/second inlets of the thermostatic valve is also small. The low temperature difference that is achieved does not enable effective control of the temperature of the electrolyte, which can vary significantly from the target value in many operating conditions.